support transformation

Cargo.toml

@@ -1,15 +1,17 @@
 [package]
 name = "struct_morph"
-version = "0.1.0"
+version = "0.5.0"
 edition = "2021"
 license = "MIT"
-description = "a (proc) macro for morphing one struct into another."
+description = "macro for morphing one struct into another."
 authors = ["shrynx <shriyansbhatnagar@gmail.com>"]
+repository = "https://github.com/shrynx/struct_morph/tree/main/lib"
+readme = "README.md"
 
 [lib]
 proc-macro = true
 
 [dependencies]
-syn = { version = "1.0", features = ["full"] }
+syn = { version = "2.0", features = ["full"] }
 quote = "1.0"
 proc-macro2 = "1.0"

README.md

@@ -1,3 +1,88 @@
-# Struct Morph
+# Struct Morph [![Rust](https://github.com/shrynx/struct_morph/actions/workflows/rust.yml/badge.svg)](https://github.com/shrynx/struct_morph/actions/workflows/rust.yml) [![Version](https://img.shields.io/crates/v/struct_morph.svg)](https://crates.io/crates/struct_morph)
 
-A (proc) macro for morphing one struct into another.
+macro for morphing one struct into another.
+
+## Installation
+
+```sh
+cargo add struct_morph
+```
+
+or manually
+
+```toml
+struct_morph = "0.5"
+```
+
+## Usage
+
+I occasionally run into use case where i have two structs representing very similar data. They share most of the fields or can sometmes even be subset of one.
+Say we have a struct `ProductRow` coming from database and another struct `ProductInfo` which is what will be sent as a json from the api.
+
+```rust
+struct ProductRow {
+    id: i32,
+    name: String,
+    description: String,
+    available_count: i32,
+    base_price: i32,
+    discount: i32,
+    created_at: DateTime,
+    updated_at: DateTime,
+}
+
+struct ProductInfo {
+    id: i32,
+    name: String,
+    description: String,
+    is_available: bool,
+    price: i32,
+}
+```
+
+now for this small when we need to convert ProductRows to ProductInfos we can manually do it, but for larger structs it becomes quite mechanical.
+
+with this library you can write the following
+
+```rust
+use struct_morph::{morph, morph_field};
+
+#[morph(ProductRow)]
+struct ProductInfo {
+    id: i32,
+    name: String,
+    description: String,
+    #[morph_field(transform = "is_available")]
+    is_available: bool,
+    #[morph_field(transform = "net_price")]
+    price: i32,
+}
+
+fn is_available(value: &ProductRow) -> bool {
+    value.available_count > 0
+}
+
+fn net_price(value: &ProductRow) -> i32 {
+    value.base_price - value.discount
+}
+```
+
+and then you can simply generate a product info from a product row
+
+```rust
+let product_row: ProductRow = ProductRow {
+    id: 10,
+    name: "The Rust Programming Language".to_string(),
+    description: "The official book on the Rust programming language".to_string(),
+    available_count: 10,
+    base_price: 50,
+    discount: 10,
+    created_at: DateTime::now(),
+    updated_at: DateTime::now(),
+};
+
+let product_info: ProductInfo = ProductInfo::from(product_row);
+```
+
+This will copy the values for fields with same name (and type) and for the rest one can provide customer transform functions.
+It does so by implementing a `From` trait for the source and target struct.

src/lib.rs

@@ -1,53 +1,140 @@
+//! This crate provides macro for transforming value of one struct to another.
+//!
+//! ```edition2021
+//! # use struct_morph::{morph, morph_field};
+//! #
+//! # #[derive(Clone, Debug)]
+//! # struct ProductRow {
+//! #   id: i32,
+//! #   name: String,
+//! #   description: String,
+//! #   available_count: i32,
+//! #   base_price: i32,
+//! #   discount: i32
+//! # }
+//! # 
+//! # #[derive(Debug)]
+//! # #[morph(ProductRow)]
+//! # struct ProductInfo {
+//! #   id: i32,
+//! #   name: String,
+//! #   description: String,
+//! #   #[morph_field(transform = "is_available")]
+//! #   is_available: bool,
+//! #   #[morph_field(transform = "net_price")]
+//! #   price: i32,
+//! # }
+//! #
+//! # fn is_available(value: &ProductRow) -> bool {
+//! #   value.available_count > 0
+//! # }
+//! #
+//! # fn net_price(value: &ProductRow) -> i32 {
+//! #   value.base_price - value.discount
+//! # }
+//! #
+//! # fn main() {
+//! #   let product_row: ProductRow = ProductRow {
+//! #     id: 10,
+//! #     name: "The Rust Programming Language".to_string(),
+//! #     description: "The official book on the Rust programming language".to_string(),
+//! #     available_count: 10,
+//! #     base_price: 50,
+//! #     discount: 10,
+//! #   };
+//! # 
+//! #   let product_info: ProductInfo = ProductInfo::from(product_row.clone()); 
+//! #   
+//! #   println!("{:?}", product_row);
+//! #   println!("{:?}", product_info);
+//! # }
+//! ```
+//!
+//! Please refer to [https://github.com/shrynx/struct_morph/blob/main/README.md](README) for how to set this up.
+//!
+//! [https://github.com/shrynx/struct_morph]: https://github.com/shrynx/struct_morph
+
 use proc_macro::TokenStream;
 use quote::quote;
-use syn::{parse_macro_input, AttributeArgs, ItemStruct, NestedMeta};
+use syn::{
+    parse::{Parse, ParseStream},
+    parse_macro_input, Ident, ItemStruct, LitStr, Token,
+};
+
+struct MorphFieldArgs {
+    transform_function_name: LitStr,
+}
+
+impl Parse for MorphFieldArgs {
+    fn parse(input: ParseStream) -> syn::Result<Self> {
+        let transform_keyword: Ident = input.parse()?;
+        let _eq_token: Token![=] = input.parse()?;
+
+        if transform_keyword != "transform" {
+            return Err(syn::Error::new_spanned(
+                transform_keyword,
+                "Expected 'transform' followed by function name",
+            ));
+        }
+        Ok(Self {
+            transform_function_name: input.parse()?,
+        })
+    }
+}
+
+#[proc_macro_attribute]
+pub fn morph_field(args: TokenStream, input: TokenStream) -> TokenStream {
+    let _ = syn::parse_macro_input!(args as MorphFieldArgs);
+    input
+}
 
 #[proc_macro_attribute]
 pub fn morph(attr: TokenStream, item: TokenStream) -> TokenStream {
-    let args = parse_macro_input!(attr as AttributeArgs);
-    let input = parse_macro_input!(item as ItemStruct);
+    let source_struct_ident = parse_macro_input!(attr as Ident);
+    let mut input = parse_macro_input!(item as ItemStruct);
 
-    if args.len() != 1 {
-        return syn::Error::new_spanned(&input, "Expected exactly one argument to morph")
-            .to_compile_error()
-            .into();
-    }
-    let source_struct_ident = match &args[0] {
-        NestedMeta::Meta(syn::Meta::Path(path)) if path.segments.len() == 1 => {
-            &path.segments[0].ident
-        }
-        _ => {
-            return syn::Error::new_spanned(
-                &args[0],
-                "Expected a single identifier as argument to morph",
-            )
-            .to_compile_error()
-            .into()
-        }
-    };
+    let target_fields = &input
+        .fields
+        .iter()
+        .map(|f| {
+            let field_name = &f.ident;
+            let transform_function = f.attrs.iter().find_map(|attr| {
+                attr.path().is_ident("morph_field").then(|| {
+                    let args: MorphFieldArgs = attr.parse_args().unwrap();
+                    args.transform_function_name.value()
+                })
+            });
+
+            match transform_function {
+                Some(func) => {
+                    let func_ident = Ident::new(&func, proc_macro2::Span::call_site());
+                    quote! { #field_name: #func_ident(&source) }
+                }
+                None => quote! { #field_name: source.#field_name.clone() },
+            }
+        })
+        .collect::<Vec<_>>();
 
     let target_struct_ident = &input.ident;
-    let fields = input.fields.iter().map(|f| {
-        let field_name = &f.ident;
-        quote! {
-            #field_name: source.#field_name.clone()
-        }
-    });
 
-    let gen = quote! {
+    let from_trait_gen = quote! {
         impl From<#source_struct_ident> for #target_struct_ident {
             fn from(source: #source_struct_ident) -> Self {
                 Self {
-                    #(#fields),*
+                    #(#target_fields),*
                 }
             }
         }
     };
 
-    let expanded = quote! {
-        #input
-        #gen
-    };
+    input.fields.iter_mut().for_each(|field| {
+        field
+            .attrs
+            .retain(|attr| !attr.path().is_ident("morph_field"));
+    });
 
-    TokenStream::from(expanded)
+    TokenStream::from(quote! {
+        #input
+        #from_trait_gen
+    })
 }

tests/tests.rs

@@ -1,35 +1,67 @@
-extern crate struct_morph;
-
 use struct_morph::morph;
 
 #[derive(Clone)]
 struct Foo {
     a: u16,
-    b: &'static str,
+    b: String,
     c: Vec<u32>,
-    d: (&'static str, &'static str),
+    d: (String, String),
 }
 
 #[morph(Foo)]
 struct Bar {
     a: u16,
-    b: &'static str,
+    b: String,
     c: Vec<u32>,
 }
 
 #[test]
-fn it_works() {
+fn simple() {
     let my_foo: Foo = Foo {
         a: 10,
-        b: "Hello",
+        b: "Hello".to_string(),
         c: vec![1, 3, 4],
-        d: ("Good", "Bye"),
+        d: ("Good".to_string(), "Bye".to_string()),
     };
 
     let auto_bar: Bar = Bar::from(my_foo.clone());
 
     assert_eq!(my_foo.a, auto_bar.a);
     assert_eq!(my_foo.b, auto_bar.b);
     assert_eq!(my_foo.c, auto_bar.c);
+}
+
+#[morph(Foo)]
+struct Baz {
+    b: String,
+    c: Vec<u32>,
+    #[morph_field(transform = "foo_d_first")]
+    e: String,
+    #[morph_field(transform = "foo_d_sec_len")]
+    f: usize,
+}
+
+fn foo_d_first(value: &Foo) -> String {
+    value.d.0.clone()
+}
+
+fn foo_d_sec_len(value: &Foo) -> usize {
+    value.d.1.len()
+}
+
+#[test]
+fn with_field_transform() {
+    let my_foo: Foo = Foo {
+        a: 10,
+        b: "Hello".to_string(),
+        c: vec![1, 3, 4],
+        d: ("Good".to_string(), "Bye".to_string()),
+    };
+
+    let auto_baz: Baz = Baz::from(my_foo.clone());
 
+    assert_eq!(my_foo.b, auto_baz.b);
+    assert_eq!(my_foo.c, auto_baz.c);
+    assert_eq!(foo_d_first(&my_foo), auto_baz.e);
+    assert_eq!(foo_d_sec_len(&my_foo), auto_baz.f);
 }